Volume controls for centrifugal fans



l July 9, 1957 J. E, MCDONALD VOLUME CONTROLS FOR CENTRIFUGAL FANS Filed March l, 1955 .6E zou mvg E m\ .mwwww United Sttes Patent iice 2,798,658 Patented July 9, 1957 l 2,798,658 VOLUME CONTRGLS FR CENTRIFUGAL FANS John E. McDonald, Newton, Mass., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application March 1, 1955, Serial No. 491,271

7 Claims. (Cl. 230-114) This invention relates to centrifugal fans, and relates more particularly to volume controls for such fans.

Centrifugal fans are usually driven by constant speed induction motors. Loads on such fans vary from time to time so that some means of varying the volumesof gas handled by the fans must be provided. Dampers may be used for adjusting gas volume, but with dampers, a reduction in gas volume does not result in a corresponding reduction in fan power consumption. The H. F. Hagen Patent No. 1,846,863 discloses a control which reduces the gas volume of a fan with a conformal reduction in power consumption, using adjustable spin vanes for spinning the gas entering a fan in the direction of rotation of the wheel of the fan. Such spin vanes and their adjusting mechanisms are relatively expensive and complicated since they must be pivoted around a fan inlet, and rotated by complicated linkage.

For many duties it is not required that the output of a fan be adjusted over a wide range, and I have found that a relatively simple and inexpensive, yet effective and ecient control can be provided by recirculating excess gas over that required by a load through a fan, and by recovering energy from the recirculated gas by passing it through fixed spin vanes which spin the recirculated gas in the direction of wheel rotation at a rate greater than the rate of wheel rotation, adjustment of the gas volume being provided by a simple damper in the recirculated gas connection.

Objects of this invention are to provide a simple and inexpensive yet effective and efcient volume control for a centrifugal fan.

This invention will now be described with reference to the annexed drawings, of which:

Fig. 1 is a side section of a centrifugal fan embodying this invention, and

Fig. 2 is an end elevation looking into the inlet of the fan, with a portion in section.

The fan has a rotor `or wheel 9 consisting of a backplate 10 secured by a hub 11 to a rotary shaft 12, a sideplate 13 and a plurality of backwardly inclined blades 14 secured to the back and side plates. The fan has a scroll-shaped casing 15 with a curved, inwardly converging inlet sheet 16 forming an axial, gas inlet passage into the wheel 9. The casing has a tangential gas outlet 18.

The construction described so far with reference to the drawings is conventional. This invention adds a recirculated gas passage 19 around the outer portion of the axial passage 17, and formed by a curved sheet 20 spaced outwardly of the outer portion of the inlet sheet 16. Spin vanes 22 are xedly attached to and between the sheets 16 and 20 adjacent the inner end of the sheet 20.

A recirculation duct 23 connects the fan outlet 18 to the inlet of the passage 19, and has a damper 24 therein. The damper has a rotary shaft journalled in the sides of the duct 23 which is connected by a crankarm 25 to the piston rod 26 of a hydraulic cylinder 27 which forms a part of a conventional load control. Upon a load reduction as indicated, for example, by a rise in steam pressure where the fan is used as a forced draft fan in a steam plant, the piston rod 26 would adjust the damper 24 towards open position. Upon a load increase, the damper would be adjusted towards closed position.

The spin vanes 22 are so shaped and placed that they spin the recirculated gas in the passage 19 in the direction of rotation of the fan wheel 9, at a speed greater than the speed of rotation of the wheel, whereby the recirculated gas aids in the rotation of the wheel so that energy is recovered by the Wheel from the recirculated gas.

The passage 19 is so located that the spinning gas emerging from it moves in a high velocity, annular stream contacting the outer positions of the fan blades 14, and blocking o such outer blade portions from contact with the primary gas stream which enters the fan wheel axially. This aids in reducing the gas volume handled by the fan since, in effect, it reduces the active blade area.

Under full load condition where the maximum volume of gas provided by the fan is required by its load, the damper 24 would be closed so that there would be no ilow of recirculated gas. Under a part load condition, the damper would be opened in proportion to the reduction in load, permitting high pressure gas from the fan outlet to ow through the passage 19 into the inlet of the fan. The fan wheel recovers energy from the recirculated gas resulting in eicient operation at reduced loads.

While one embodiment of this invention has been described for the purpose of illustration, it should be understood that the invention is not limited to the exact apparatus and arrangement of apparatus illustrated, since modications thereof may be suggested by those skilled in the art, without departure from the essence of the in- Vention.

What is claimed is:

1. In combination with a centrifugal fan having a fan wheel, an axial, primary gas inlet passage to the wheel, and a gas outlet, means forming a secondary gas passage extending around said inlet passage and having an outlet discharging into said inlet passage, means forming a gas recirculation passage connecting said rst mentioned outlet with the inlet of said secondary passage, and spin vanes in said secondary passage adjacent said second mentioned outlet for spinning the gas entering the inlet passage therethrough in the direction of Wheel rotation.

2. The combination claimed in claim 1 in which an adjustable damper is provided in the recirculation passage between the first mentioned outlet and the spin vanes.

3. The combination claimed in claim 1 in which a pair of spaced-apart, inwardly converging sheets form said secondary passage therebetween, the innermost 0f said sheets forming said axial passage, the outermost of said sheets extending into said inlet passage.

4. The combination claimed in claim 3 in which an adjustable damper is provided in the recirculation passage between the first mentioned outlet and the spin vanes.

5. The combination claimed in claim 4 in which the inner end of the outermost sheet terminates outwardly of the inner end of the innermost sheet, and in which the spin vanes are arranged in the secondary passage adja cent the inner end of the innermost sheet.

6. The combination claimed in claim 1 in which the inner end of the secondary passage terminates outwardly of the inner end of the axial passage, and in which the spin vanes are in the secondary passage adjacent its inner end.

7. The combination claimed in claim 6 in which an adjustable damper is provided in the recirculation passage between the first mentioned outlet and the spin vanes.

References Cited in the lle of this patent UNITED STATES PATENTS 1,846,863 Hagen Feb. 23, 1932 2,327,841 Hagen Aug. 24, 1943 2,393,042 Hagen Jan. 15, 1946 2,476,543 Geissler et al. July 19, 1949 2,488,267 Cassie Nov, 15, 1949 

